In any convoy of utility vehicles or other applications wherein a group of vehicles are in close proximity, intra-convoy voice communications and the ability to efficiently transfer data among the vehicles are imperative for safe and efficient operations. This is especially true during operation of convoys of military utility vehicles. One method of real-time, direct voice intra-convoy communications is via individual communication devices, such as walkie-talkies, used within the convoy. Other more advanced concepts may incorporate satellite communications from one or more vehicles within the convoy. Such satellite communications can be hampered by the number of satellite-equipped vehicles, and usually is restricted to satellite text message systems. Furthermore, since the signal must be bounced off a satellite orbiting the earth, there is an inherent delay in the text message communications. This time delay could prove critical to safe convoy operations.
One of the factors that inhibit communication systems in convoys is the lack of a fixed infrastructure of nodes and antennas in the areas where convoys operate. Attempts have been made to overcome this problem by establishing communication via mobile ad-hoc networks (MANET). A MANET includes a number of geographically-distributed, potentially mobile nodes sharing one or more common radio channels. MANETs differ from other networks (e.g. Internet, wireless LAN), because a MANET lacks a fixed infrastructure. Instead of having centralized routers/servers and local access points, the network consists of only nodes and each node behaves as a router and forwards packets through the network. A MANET can be created and adjusted on the fly as the nodes enter and exit the network. MANETs are more difficult to administer than a fixed infrastructure network, but there are many situations in which a fixed infrastructure network is impractical or unavailable, for example, after a natural disaster.
MANETs pose many challenges to current designers. Due to the lack of a central controller and processor, all functions must be distributed amongst the nodes. As a result, all nodes are essentially the same in their construction. Additionally, in a wireless setting the distance between two nodes may be greater than the radio transmission range of the nodes. This forces information to be hopped through other nodes to get to the destination node. As the network grows in size, routing the information through the nodes becomes more complex. As a result, much effort has been put into the design of routing protocols. The routing protocol for a network can greatly affect the speed and quality of a MANETs service. Routing protocols must adapt to the frequent changes in the network, and often must do so with information that is not current with network activities. Further, gathering new information about the network puts a strain on network resources and may not update frequently enough to be effective.
One of the greatest difficulties confronting ad hoc wireless networks is route recovery after a route breakage occurs. Often breakages are the result of two nodes on a route losing communication with each other. Loss of communication can result from one of the nodes leaving the network, or when the distance between nodes becomes longer than the transmission range of the nodes. Physical barriers, interference, and other natural phenomena can also interrupt the communication path. Two opposing protocol designs have been developed to improve route recovery, one proactive and one reactive.
The proactive protocols require each node to maintain a current routing table with routes to every other node in the network regardless of whether any data transmission will occur with the other nodes. Proactive routing protocols have a short latency for discovery of a route, because a source node already has the route to a destination node in its routing table. Maintaining a current routing table, however, causes proactive protocols to use a considerable amount of network resources. Nodes are continually sending packets around the network as they verify routes to ensure their routing tables are up to date. Exemplary proactive protocols are Open Storage Path First (OSPF), Optimal Link State Routing (OLSR), and Topology Broadcast based on Reverse-Path Forwarding (TBRPF).
Reactive protocols require a node to maintain information only on current or recently used routes in which the node itself is involved. Under these protocols, a source node initiates route discovery only when the source node needs to send packets to a destination that is not on a current route the source node is using. Thus, network resources are not unnecessarily tied up discovering routes that may never be used. The route discovery process, however, will incur a large latency during startup of the route because a new route must be discovered before data can be sent. Reactive protocols generally show better bandwidth efficiency than proactive routing if there are a small number of source-destination pairs. Exemplary reactive protocols are Ad hoc On-demand Multi-path Distance Vector (AOMDV) and Dynamic Source Routing (DSR).